The present invention relates to the use of particular polyelectrolytes of low molecular weight and inorganic salt dispersant materials to achieve a reduction in the water demand of finely divided solids in water. It relates particularly to this use as an improvement in wet process production of portland cement.
It is known that polyelectrolytes such as polyacrylic acid salts, copolymers of acrylic acid and acrylamide, hydrolyzed polyacrylonitrile and the like which are flocculants at higher molecular weights show different properties and act as dispersants at lower molecular weights. Such polymers having molecular weights from a few thousand up to about 50,000, for example, have been recommended for use in various dispersant applications. See, for example, U.S. Pat. Nos. 3,534,911 and 3,604,634, relating to the grinding of calcium carbonate.
Combinations of polyacrylates and certain inorganic compounds have also been used in forming foundry sand compositions and in clay benefication and drilling fluids and muds. For example, polyacrylic acids and alkali metal salts, e.g., sodium carbonate, sodium silicate and the like are disclosed in U.S. Pat. No. 2,702,788 as resulting in increased drilling and viscosity and, hence, an increased yield of mud is obtained. Typically, the polyacrylates are used in amounts of about 0.1% or more and the sodium carbonate in amounts of from about 3%, or more. See also U.S. Pat. Nos. 3,583,911, 2,552,775 and 3,220,946, which generally disclose similar applications. The '911 patent teaches such use of polyacrylic acid and methylcellulose graft copolymers with from about 0.5 to about 7% by weight alkali metal salts while the '775 patent teaches the use of from about 0.2-5% by weight polyacrylates with from 1-12% alkali metal salts for similar applications and as agents for preventing water loss from the drilling fluid to the formation. The '946 patent discloses a synergistic action in clay benefication uses with mixtures of polyacrylates of M.W. 25,000 to about 150,000 and specific inorganic salts. Certain salts, e.g., sodium chloride, sodium carbonate, calcium carbonate, etc., are disclosed as being ineffective in said combination. U.S. Pat. No. 3,923,717 discloses the use of styrenemaleic anhydride to reduce water demand of wet process portland cement slurries. A two-part study by the Portland Cement Association, Chicago, Ill., entitled "Slurry Thinners". (Part I, Clausen et al., May 1953, Part II by Dersnah, March 1955) discloses the evaluation of various inorganic salt dispersants and mixtures thereof with other surface active agents in reducing the water content of cement slurries in wet process applications.
Of the above prior art references only the '717 patent and the Portland Cement Association Study relate to the wet grinding process for making portland cement, wherein limestone and clay and, optionally, a small amount of iron oxide are ground in the presence of water to obtain a slurry of very fine particles. Such slurry is then fed into a high temperature kiln where it is dried and calcined to form the clinker which is then ground to make portland cement. The water demand of the finely ground limestone-clay slurry is fairly high and usually requires a relatively large proportion of water, for example, about 30-50% by weight, usually from about 25 to about 35%, to obtain a fluid, pumpable slurry. This limits the rate at which the slurry can be processed and fed to the kiln and it also requires a large fuel input to dry and calcine a given quantity of solids to the clinker stage.
Many substances with dispersant activity are available and have been tried in order to decrease the water demand of suspended inorganic solids in various high solids water suspensions or slurries for various applications. Most of these, particularly in the wet process for making portland cement, have proven relatively ineffective or undesirable for one reason or another. Complex phosphates are undesirable because they tend to hydrolyze at the warm temperature developed during grinding and in storage of the slurry and because of the adverse effect of residual phosphate on the properties of the final portland cement product. Lignosulfonates, alone and in combination with inorganic salt dispersants, have been tried for this use, but these requires high addition levels for only marginal improvement. They also lose their activity rapidly during storage of the slurry. Many materials are also too expensive to be economically utilized in such operations.
We have now found, however, that the water demand of suspended inorganic solids necessary to make a pumpable slurry in the wet grinding process for making cement can be significantly reduced by use of water-demand reducing agent combination as described below.